BUG #15596: Folders created with wrong permissions when installing an extension with a non-default umask

The following bug has been logged on the website:

Bug reference: 15596
Logged by: Antoine Amarilli
Email address: a3nm(dot)postgresql(at)a3nm(dot)net
PostgreSQL version: 11.1
Operating system: Debian testing
Description:

Hi,

I was trying to install a PostgreSQL extension (namely,
https://github.com/PierreSenellart/provsql) This extension uses the macros
in the file /usr/lib/postgresql/11/lib/pgxs/src/makefiles/pgxs.mk to install
itself, so running "make install" does the following:

/bin/mkdir -p '/usr/lib/postgresql/11/lib'
/bin/mkdir -p '/usr/share/postgresql/11/extension'
/bin/mkdir -p '/usr/share/postgresql/11/extension'
/bin/mkdir -p '/usr/share/doc/postgresql-doc-11/extension'
/usr/bin/install -c -m 755 provsql.so
'/usr/lib/postgresql/11/lib/provsql.so'
/usr/bin/install -c -m 644 .//provsql.control
'/usr/share/postgresql/11/extension/'
/usr/bin/install -c -m 644 .//sql/provsql--1.0.0-dev.sql
'/usr/share/postgresql/11/extension/'
/usr/bin/install -c -m 644 .//doc/provsql.md
'/usr/share/doc/postgresql-doc-11/extension/'
/bin/mkdir -p '/usr/lib/postgresql/11/lib/bitcode/provsql'
/bin/mkdir -p '/usr/lib/postgresql/11/lib/bitcode'/provsql/src/
...

However, the umask of my user is 077, so these "mkdir -p" invocations are
creating folders that can only be read by root, which is not the intended
behavior.

Would it be possible to fix this by redefining MKDIR_P="mkdir -m 0755 -p" in
pgxs.mk so that the mkdir invocations create the folders with the right
permissions, in line with the "install" invocations? Thanks!